Replace Microsoft.Language.Xml with the modern GuiLabs.Language.Xml library

[Meir017]

this should be migrated to https://www.nuget.org/packages/GuiLabs.Language.Xml/

Originally posted by @Meir017 in #116

[tintoy]

Glad to see it's from the same author; hopefully that means it won't be too hard to switch over (it's all abstracted away by XSElement, XSAttribute, etc, so at least there's a well/described contract for the required functionality). I'm happy to get have a go at this next week 🙂

[Meir017]

it's also managed in the same repo :) (they changed the name here KirillOsenkov/XmlParser@cba26d0)

I started looking into this a bit and got lost a bit in the abstraction - the main change is that the XML library migrated to use the Roslyn green/red nodes separation and removed the base class src/Microsoft.Language.Xml/Syntax/XmlElementSyntaxBase.cs (which affects the abstraction we have here where all types have some basic properties).

KirillOsenkov/XmlParser@bbfe132#diff-9886154c3e5921e6a7b8d74fb51d77fcaa8240eaebe54549f5899f903b5d7c85